The invention relates to an apparatus for producing a variable volumetric flow in a fuel feed system, especially for use in common-rail injection systems.
In common-rail injection systems, it is necessary to pump the fuel out of a tank, to compress it and to hold the compressed fuel ready for injection by injectors in a pressure reservoir referred to as a rail. The pressure in the rail and the quantity of fuel removed from the rail by injection vary with the operating conditions of the engine.
In order to enable the pressure in the rail and therefore also the volumetric flow delivered to the rail to be influenced in an appropriate manner, use is made, for example, of a configuration described in European Patent Applications 0 643 220 and 0 643 221, corresponding to U.S. Pat. Nos. 5,427,066 and 5,746,180. An upstream feed pump draws the fuel from a tank through a filter and supplies the high-pressure pump. The compressed fuel is stored in the rail and injected into combustion chambers by the injectors. In that method, use is made of constant-displacement pumps, which deliver a fixed volume with each revolution of the shaft. A variable delivery rate of the system is achieved by discharging the unrequired but already compressed volumetric flow with the aid of a valve. However, that principle is not advantageous for high-pressure systems in terms of energy.
In terms of energy, preference should be given to systems in which the pump delivers only a volumetric flow limited to the quantity that is actually required. A method known from low-pressure systems, in which the volumetric flow is influenced through the use of the adjustability of the volume of the displacer elements is disadvantageous in the case of diesel injection systems because of the high mechanical outlay and the large control forces that are required. Pumps with constant-volume displacer elements are accordingly more advantageous. In pumps with constant-volume displacer elements, the variable volumetric flow is achieved by differences in the filling ratio of the displacer volumes. One possibility, for example, is to force the volume that is not required in piston pumps out of the initially completely full cylinder before compression begins. The fuel can be forced back into the inlet line or into an additional bypass with the aid of a controllable valve. The disadvantage therein is that a quick-operating valve is necessary for each displacer element.
Instead of filling the cylinder completely at the outset and releasing the quantity that is not required, it is also possible to vary the filling level from the outset by limiting the supply to the displacer elements.
One possibility for limiting the supply is to throttle the entire volumetric flow fed to the pump or the volumetric flow fed to each individual displacer element. In that case, use is made of adjustable throttle valves, which allow proportional variation of the volumetric flow by variation of the throttling cross section. The maximum cross section of the throttle valve is set for the maximum volumetric flow at full load and rated speed. The interaction between the maximum volumetric flow that can be delivered, which is dependent on the rotational speed of the pump, and the adjustability of the throttle valve, yields a relationship between the adjustable volumetric flow and the manipulated variable as a function of the rotational speed of the pump. At low rotational speeds of the pump associated with a low maximum volumetric flow that can be delivered, the useful adjustment range of the throttle valve is severely limited since only a small area of the throttling cross-sectional area can be used with throttling effect. It is only at maximum rotational speed that the full adjustment range of the valve can be exploited. If, for example, a pump is to be operated at a rated speed of 3000 rpm and a delivery rate of 0.5 ml per revolution, the throttle valve must be constructed for a maximum volumetric flow of 1500 ml/min. At a speed of 300 rpm and the maximum volumetric flow delivered resulting therefrom of 150 ml/min, only 10% of the adjustment range of the throttle valve is used for control between zero load and full load.
Another disadvantage of known high-pressure piston pumps with a plurality of cylinders and central limitation of the volumetric flow being fed in, is the outlay associated with ensuring uniform delivery-flow pulsation. Due to the finite number of displacer elements, there is always pulsation in the delivery flow from the pump, due to the very principle involved. The fluctuation in the delivery flow about a mean value, which is referred to as pulsation for short, results from the superposition of the component delivery flows coming from the individual displacer elements to give an overall delivery flow. The minimum pulsation for a fixed number of displacer elements is obtained if all of the displacer elements have the same component delivery flow. Therefore, in the case of piston pumps, it is a matter of filling each individual cylinder equally. If the supply is limited centrally and distribution between the individual cylinders takes place downstream, agreement between the characteristics of the inlet valves in particular is responsible for an equal filling ratio of the individual cylinders and therefore for uniform delivery flow pulsation. Differences in the characteristics of the inlet valves and the resulting differences in the flow rates are particularly noticeable at partial load in the form of nonuniform delivery-flow pulsation. Producing valves with the same inlet valve characteristic is extremely expensive since it is difficult especially to produce springs of identical length required for the inlet valves.
It is accordingly an object of the invention to provide an apparatus for producing a variable volumetric flow in a fuel feed system, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known apparatuses of this general type and in which it is possible, by limiting the supply, to adjust the delivery flow to all volumetric flows required between zero and full load at any rotational speed between an idling speed and a rated speed. With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for producing a variable volumetric flow in a fuel feed system, comprising a high-pressure pump having at least one cylinder, at least one piston respectively moveable in the at least one cylinder, a shaft driving the at least one piston, and at least one inlet defining an angle between opening and closure of the at least one inlet; and a metering device disposed at the at least one inlet of the high-pressure pump, the metering device synchronized with the shaft and feeding a variably meterable volume of fuel to the at least one cylinder during each revolution of the shaft, and the metering device performing metering by varying the angle between opening and closure of the at least one inlet.
The advantage of the invention is that an adjustment range which is constant for all rotational speeds of the pump is ensured for the purpose of influencing the variable volumetric flow. Moreover, uniform, low delivery-flow pulsation is obtained by ensuring uniform distribution of the overall supply to the individual displacer elements. Finally, it is advantageous that the high-pressure pump can be operated at a very low feed pressure.
In particular, the invention makes provision for a predefined, meterable volume to be fed to the delivery elements of the high-pressure pump during each revolution of the pump. The resulting variable filling ratio of the displacer elements thus results in a volumetric flow that can be varied between zero and full load. At the same time, the adjustment of the filling ratio is decoupled from the rotational speed of the high-pressure pump. Consequently, the adjustment range for influencing the filling ratio is the same size for all rotational speeds. Metering is effected by varying the angle between the opening and closure of the inlets of the individual cylinders in synchronism with the angle of the shaft and thus as a function of the position of the pistons of the high-pressure pump. Preferably, the inlet to the cylinder of the high-pressure pump is always opened in the region of the top dead center position of the piston and closed at any desired piston position down to the region of the bottom dead center position. As a result, the cylinders fill with a defined volume that determines the filling ratio.
In particular, a continuous volumetric flow is fed by an upstream feed pump into the shaft, which also drives the pistons through cams or eccentrics, for example. The volumetric flow is distributed by passages and external grooves. A perforated sleeve is disposed in an axially adjustable manner on the shaft. The shape of the grooves on the shaft and the apertures in the sleeve are configured in such a way that different opening and/or closing angles are obtained, depending on the axial position of the sleeve. The shape of the apertures and grooves can also be interchanged. The shaft and the sleeve are situated in a casing with further passages for carrying the volumetric flow away to the individual cylinders. Moreover, the inlet and the outlet can also be formed in any desired combination in the casing and/or the shaft. In other words, both the inlet and the outlet are accommodated in the shaft or in the casing or one of them is situated in the casing and the other is situated in the shaft.
For example, by virtue of a slot-shaped configuration of the grooves and apertures and a specific orientation of the grooves and apertures, the metering of the supply and therefore opening begins when the groove and the aperture begin to overlap. The interruption of the supply (closure) takes place when the groove on the shaft has run out of overlap with the aperture in the sleeve. The instant of opening advantageously remains constant, given an orientation of the groove parallel to the axis.
Axial displacement of the sleeve changes the phase length of the overlap between the grooves and the aperture (angle). Combining these overlaps with the position of the piston allows a defined volume to be admitted to the cylinders. The axial displacement of the sleeve represents the adjustment range, which is the same size for all rotational speeds. For all rotational speeds, it is possible to operate with the same adjustment range between zero delivery (no overlap between the grooves and apertures during a complete revolution of 360xc2x0 and full delivery (maximum phase length of the overlap of 180xc2x0, that is to say throughout the entire time during which the piston is moving from the top dead center position to the bottom dead center position). For the purpose of varying the angle, the closing instant can be selected to be fixed and the opening instant variable with respect to the shaft, or the opening instant can be selected to be fixed and the closing instant variable, or both can be selected to be variable. For example, the inlet to the cylinder or cylinders in each case can be opened at the top dead center position of the respective piston and closed at any desired position of the piston down to the bottom dead center position, in parallel.
Instead of a single xe2x80x9ccontrol slotxe2x80x9d, it is also possible in the same way to use a plurality of xe2x80x9ccontrol slotsxe2x80x9d in the form of grooves, apertures etc. on a sleeve, disk or core or a plurality of sleeves, disks or cones, each with a xe2x80x9ccontrol slotxe2x80x9d in order to serve a plurality of pistons in parallel. Instead of a combination of a shaft, a displaceable sleeve and a casing, it is also possible in the same way to use only a combination of a displaceable shaft and a casing or a combination of a shaft and a displaceable casing. It is also possible to use conical or disk-shaped bodies or bodies shaped in some other suitable way instead of shafts and/or sleeves and/or casings.
Moreover, it is also possible for the inlet and the outlet to be interchanged in all embodiments. In other respects, the statements applicable to the displaceable sleeves also analogously apply to the other embodiments.
All of the lines and passages and cross sections of the overlapping grooves and apertures are constructed to be unthrottled at maximum rotational speed for maximum volumetric flow. Thus, limitation of the volumetric flow does not result from throttling but only from variation of the phase length of the opened inlet to the displacer elements. A high feed pressure is thus not necessary by virtue of the principle involved. The low feed pressure also results in lower leakage.
Another advantage of the metering of the supply for each individual cylinder in accordance with the invention, as compared with throttling of the entire pump supply, lies in the improvement in uniform distribution of the supply to the individual cylinders. The uniformity of distribution to the individual cylinders is dependent only on matching of the geometry and configuration of the grooves and apertures. The geometrical tolerances required for this purpose are significantly easier to comply with than, for example, the tolerances of spring characteristics for identical inlet valves.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an apparatus for producing a variable volumetric flow in a fuel feed system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.